Conventional integrated circuits operate on dual voltages, the lower voltage being in the core side in the range 1.8 v, 1.2 v or 1.0 v. External voltages are available in ranges of 5 v, 3.3 v, 2.5 v, 1.8 v, 1.2 v. An existing method for supplying different ranges of external supply voltages for a single IC is to turn on a step down regulator (switching or linear) to supply the low voltage core logic of the IC if the external voltage is higher than required by the core. A switch can be turned on to supply the core if the external supply is in the same range as the required core supply. If the external supply is less than the required internal core supply, a boost-switching regulator can be turned on. All these options can be exercised by options set on the application board for a dedicated external supply.
Patent number FR2838840, which is incorporated by reference, suggests using a comparator for comparing the supply with a reference and take decision accordingly. The comparator is kept on all the time, but the operator IC generates lots of noise and may cause erroneous switching of the comparator and may cause lots of noise on the internal supply line, especially, when two ranges of the supply are so that the lower limit of the higher supply range is relatively close to the higher limit of the lower supply range.
Furthermore, during the power up phase, when the decision-making circuit is not fully activated, then an erroneous supply-management decision, however momentary, may expose low-voltage components to a high voltage.